... (I'm
checking the md5sum of /dev/cdrom and this comes up with the wrong
number and an error message)
...

I burned my CDs using cdrecord for the commandline (which shouldn't
make any difference) and saw the same sort of behavior you report.
There was very peculiar constency to the behavior though. It was always
on binary iso 3 and the rescue disc iso. As I was bugzilla-ing that, I
noticed that not only were the md5sums consistently bad on those iso's,
but the bad md5sum I was geting was always the sam, independent of
CD/iso. Further, when I used the checkdisc thingy embedded in binary
disc 1, it gave the coasters a clean bill of health. When I stuck the
coasters back into my burner and md5sum'ed the device again, I got
correct md5sums! Then I burned another CD, md5sum'ed it (bad), ejected
it, re-inserted it, and md5summed again (OK). I conjecture that burning
those particular iso's was leaving my Yamaha cd-rw in some inconsistent
state.

So, my advice is: don't despair. The CD's may well be OK. Try an
independent check, like md5summing in a different drive, or using the
diskcheck utility.